Variable gain voltage amplifier



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VARIABLE GAIN VOLTAGE AMPLIFIER Filed Dec. 20, 1946 2 Sheets-Sheet 2 Tin-7.

I l IN VEN TOR.

@gli BY @04u/f A TTORNEY Patented Feb. 14, 1950 VARIABLE GAIN VOLTAGE AMPLIFIER George B. Loper, Dallas, Tex., assignor, by mesne assignments, to Socony-Vacuum Oil Company, Incorporated, New York, N. Y., a corporation of New York Application December 20, 1946, Serial No. 717,557

1 claim. l

This invention relates to automatic gain control systems and particularly to automatic gain control systems suited for seismic amplifiers.

In seismic prospecting systems, seismic waves generated by detonation of an explosive charge and reflected from subterraneous strata are converted to electric signals by a spread of geophones connected through amplifiers to a multi-trace oscillograph or equivalent recorder. From the resulting record, a diagram or map of the strata or interface contours may be plotted. For accurate determination of the time intervals, the amplifier gain is initially high so to insure a sharp break in the record indicative of the time of arrival of the first direct traveling waves. Subsequent to reception of the initial direct Wave, a series of reflected waves are received from the various subterranean interfaces, the earlier of the reflected waves being of the higher amplitude. To obtain a record in which the traces of the earlier refiected Waves do not overlap and in which the traces of later refiected waves are of reasonably large amplitude, the gain of each of the several amplifiers is reduced immediately upon or shortly after reception of the first direct waves and then is increased as a function of time or of the received strength of the signal.

The variations of gain, as effected in previously used amplifier systems, produce spurious signals or modulation effects whose rate of variation with time is of the same order as that of frequencies of seismic Wave signals. Consequently, the spurious signals cannot be selectively attenuated by filter circuits, an expedient often employed for that purpose in radio receivers whose automatic gain control voltage is derived by rectification of the relatively high-frequency carrier voltage.

In accordance with the present invention, the gain of an amplifier may be continuously varied over a Wide range and at slow or rapid rate preferably in dependence upon the magnitude of the signal being amplied'without creating spurious signals of appreciable or disturbing magnitude. More specifically, the gain control voltage is applied to the suppressor grids of two pentode tubes, one serving as the signal amplifier and the other as a stabilizer tube, and the anode and screen grid circuits of the two thermionic tubes are so interconnected that the tendency of the average plate' potential of each of the ltubes to change with Variation of its suppressor grid potential is minimized by the concurrent change of the screen grid potential of the other of the tubes. In effect, by use of the stabilizer tube, the suppressor gridplate transconductance of the amplifier stage is 55 2 made closely to approximate or to equal zero throughout the operating range so that variation of the suppressor-grid potential for control of amplification of signals by the amplifier stage does not result in appearance of spurious signals of appreciable magnitude in the amplifier output.

The invention further resides in features of combination and arrangement hereinafter described and claimed.

For more detailed understanding of the invention, reference is made to the accompanying drawings, in which:

Fig. 1 schematically illustrates an amplifier stage of conventional type.

Figs. 2 and3 are explanatory figures referred to in comparison of operating characteristics of conventional amplifiers and those embodying the invention;

Figs. 4, 5 and 6 schematically illustrate amplifier stages embodying the invention; and

Fig. 7 schematically illustrates a geophone amplifier channel.

For more complete understanding of the main source and nature of spurious signals arising in the control of gain of -a conventional pentode amplifier stage, reference is made to Fig. 1 in which an alternating signal voltage Egl to be` amplified is applied to the control grid gl of the tube I0. Ihe anode or plate a of the tube is connected through a load impedance, such as resistor Il, to the positive terminal l2 of a suitable source of direct current such as a battery or power supply Whose negative terminal (B-) is grounded. The amplifiedsignal voltage across the load resistor Il may be applied to a subsequent stage or to a translating device through the coupling capacitor I3. The screen grid g2 of the tube l0 is connected to the positive terminal I4 of a source of direct current; for convenience terminal Ill may be an intermediate tap of the aforesaid battery or equivalent power supply used to supply the anode current. The suppressor grid g3 of tube I0 is connected to a source of directcurrent Voltage E93 to be varied for control of the gain of the amplifier stage. 'Ihe negative biasing voltage for the control grid gl may be and preferably is provided by cathode-resistor 9.

As shown by curve G of Fig, 2, as the potential E93 of the suppressor grid is made more negative, or less positive, the signal gain is reduced and may in factv be Varied over a very wide range. Otherwise stated, the control grid-to-plate transductance of the tube may be decreased by making the potential,v of the suppressor grid more negative with .respect to the cathode of the tube.A

The effect of variations offthefgan control voltage Eg3 upon the potential of the output coupling point I is evident from curvefE off Fig. 3: for example, a largexand rapid change 4 of the negative suppressor voltage Would causeA correspondingly large and rapid changes of the positive potential E of tl'iepoint7 I5:v These;

changes because not appearing in the impressed voltage Eg! are spurious signals and therefore undesirable componentsof the amplifier output voltage. Y

Whether expressed in terms of output current I'or of potential E of the coupling point, these spurious'signals are not serious when' the rate of-`change of the gain control voltage Eg3 cor# responds with a frequencyl which is lowv compared with thev lowest signicant frequencyV of `the-signal voltage Egl. When all the signal frequencies are low, as in seismic amplifiers, or when some of theY signal frequencies are low, asin high-fidelity'audio systems, theY spurious signals cannot be minimized by frequency-selective networks `or filters kbecausetheir frequency `components are of the same order as that of rlegitimate signals. It is to the solution of this problem that the present invention is directed..

The probleinisf'conventionallyl solved by use of push-pull circuits, sometimes knownv as balanced-modulators whenused in this connection. This method usually calls for relativelylarge high-delity transformers or` other additional electronic components not needed -in the Vpresent invention. Furthermore, the advantage of the single-ended amplifier over the push-pull, where closelyassociated variable lters and volume controls are to be used, is apparent tothose skilled in the art, as lters and volume controls for' push-pull are of more involved design and call for more electronic components.

For purpose of direct vcomparsion, the ampli-I fier stage of Fig. 4,' whichA embodies the presenty invention, is the'same as that of Fig. 1 above discussed except forv addition ofthe stabilizerVV tubeiI and its connections. The suppressor grid. G3:of stabilizer tube AIIS isconnected .to the supi presser grid g3 of the signal amplier'tube IIl so-that the gain control voltage Eg3 isv concurrently and similarly eective upon the suppres-v sorgrids of both tubes; The anode a of the amplifier tube I0 is connected to the screengrid G2'of the stabilizer tube so` that the load current traversing impedance II is theralgebraic" sum of the anode current ofxtube Ill` and the. screen grid current'of .tube IB, andthelaverage direct-current potential of coupling pointl I5 is` jointly determined by the anode current of tube IB and the screengrid current of tube I6. The' screen'grid g2 of. the amplier tube IIJ s'connected to the anode-A of the stabilizer tube I6. The negative biasing voltage for the control grid GI-` of the stabilizer is preferably provided 'by a resistor I'I connected betweencathode K and the chassis or ground.

With the anode and screen grid circuits of tubes Il J and I 6 so interconnected, variation'of the suppressor voltage E93 is effective, as described above, to control the signal amplication generally in accordance with curve G of Fig. 2 but the average load current, instead of varying widely in manner indicated by curve I of Fig. 2 `and as characteristic of the unstabili'zedcircuiti ofi Fig. 1, remains; substantially constant assindicated by'any of curves I1, I2, or I3 of Fig. 3. These are typical load-current lcurves vfor the stabilized circuit of Fig. 4, the

particularshape-l depending upon the relative values chosen for the cathode resistors 9 and Illof thetwoftubes-.f`

YParaplirasedi151fterrns of the potential at the coupling-'point' I5,l variation of the suppressor voltage Eg'3"` changes the signal gain generally in accordance-Withicurve G of Fig. 2, but the average direct-current potential of the coupling point I5 Ainstead of varying widely in manner indicated by curve E, Fig. 3, and as characteristic of the unstabiliz'ed f circuit'rcf Fig; 1,= remains f substantially constantxa's.4 indicated Lb'y any fone` of i curves E1, E2, or E3,. Fig.' 3;. Thesearei typical `voltage curves -ofi pointV I5-2for the stabilized circuit of Fig. 4,1 the particular.` shape `depending 1 upon the selectedrelative values. of tnel cathode resistors= 9 and.: Il; With the stabilized. circuit, the. gain control voltage fappearingin the: anodescircuiticf the signal amplifier; tube IU is .opposed 'by a cor-` responding" voltage', intth'el 'screen'circuit :..of tube IB; which isz180? `out'.of:phase.. These two"vo1t'- ages substantiallylbalancescouti. withV the. result; above stated, that. ther potential :ofpoint` I5-'1 is substantially independent of'n variationsf. of gain control :voltage :E93:

From considerationiof thezcurves'Il, I2, Iscr Fig. 2 -or curves E1, Ez, EafofiFigz, itisevident thegainfof` the amplier -sta'gefas far 'as variations fof voltage E93,- thev 'suppressor' voltage, are concerned is .veryf-lowland in factliszeraor 4nearly so throughout=thefrange for Whiclfruthe supprese sor grid yvoltage Vis va-ried'. substantially to Vchange thegain of the-stageforthe signal voltagepEgL Otherwise stated, the control grid-platetransconductance ofi the; stage can be.v varied' over 1 a' wide rangev by variation of the suppressorA grid voltage Eg3-but becauseof the yaforesaidinterconnection yofithe anodes and 'screengridsl of the tubes the suppressor grid-plate transconductance is =and remainsanegligible'; so greatly minimizing oravoiding spuriousfsignals otherwise inherent-` ly. arising from the-variationsV of I the suppressory grid voltage; Byeway-'of examp1e,,the.-following parameters `aregiven:

Resistor I I: 250,000 ohms. Resistor 9: 2.500 ohms Resistorv I'I: 2,-500.ohms` www G2 will varyfandl soalso-will-the potential of plate ahvary, but the variations at the screenplate'idiagonal of the bridge will be 180' out of phase and the potential at point I5 .will besubstantially constant-with respect to changes of Eg3. This method affords a means of attaining optimum constancy of the potential at I5 by careful vselection of resistors I8 and I9.

The circuit shown in Fig. 6 is basically the same as that of Fig. 4, differing only in that the screen grid g2 of the tube I0 and the anode A of tube I6 obtain their operating voltage through a series resistor 2U connected to the positive terminal I2 of the power supply rather than from an intermediate tap I4 of the battery or other power supply. To avoid undesirable feedback from the anode A to screen grid g2, there should be provided a large bypass condenser 2|, having for example a capacity of microfarads. This method of providing the screen grid voltage is practical with the stabilized circuit because the anode circuit of the tube I6 regulates the voltage of the screen grid of the amplifier tube whereas in the conventional circuit, Fig. 1, the series resistor method of obtaining the operating voltage for screen grid g2 would not be satisfactory because the screen grid Voltage would vary substantially with Variations of the suppressor grid voltage and so change the operating conditions of the tube to produce or enhance distortion of the signal.

In Fig. 7 there is shown a seismic signal amplifier embodying two stabilized amplifier stages each generically similar to those of Figs. 4-6 and specifically the same as that of Fig. 6. In brief, a seismic wave detector or geophone 22 is coupled by transformer 23 or equivalent to the control grid circuit of the first amplifier tube I0. cluded selectively to pass the seismic signal frequencies which lie in the range of from about twenty to about eighty cycles and markedly to attenuate frequencies which are outside of that range: the frequency characteristics of these filters may remain fixed during a recording run or may vary automatically as disclosed in copending application, Serial No. 679,991 of John P. Minton.

The output of the first stage comprising the signal amplifier tube I0 and its stabilizer tube I6 is coupled to the control grid circuit of the second signal amplifier tube IOA preferably through a. second filter network 24A also suited to favor signals in the seismic frequency range. The output of the second stage comprising the signal amplifier tube IDA and its stabilizer tube ISA may be coupled to a conventional output amplier stage. This is in turn coupled in usual manner to a multi-trace oscillograph recorder 25 and to a rectifier 26 which derives from the amplified geophone signal a direct-current voltage Whose magnitude is a function of the signal amplitude and which may be utilized automatically to control the gain of one or more of the amplifier stages. A preferred and satisfactory amplifier-rectifier arrangement for deriving control voltage E93 is disclosed in my co-pending application, Serial No. 585,095. The filter 2'I reduces the ripple-frequency component of the direct-current output of rectifier 26: the time constant of this filter network may be fixed at low value to insure rapid reduction of gain upon arrival of the first direct waves at the geophone or the time constant may be automatically varied as disclosed in co-pending application,

A filter network 24 is preferably in- 6 Serial-No. 708,331,0f William B. Hemphill. In any event, vthe desired rate of change of the automatic gain control voltage Eg3 may be such that except for the stabilizing action of tubes I6 and IBA, spurious signals would appear in the output of the amplifier tubes I0, IIIA and' these signals, because having frequency components similar to those of the geophone signals,`

would be passed by the filter 24A and coupling condensers I3 and I3A and ultimately appear in the recorder trace as a component which interferes with recognition of the legitimate signals. With the stabilizer tubes, however, change of the gain control voltage E93 produces no spurious signals of any consequence regardless of whether the gain control voltage be changed slowly or rapidly. Therefore, the record of the signals produced by recorder 25, or equivalent, represents the seismic wave signals substantially free of distortion or masking by spurious signals incident to the automatic gain control action.

With the switch 28 in the #2 position, the automatic gain control voltage E93 is applied to control the gain of one of the stabilized stages and the gain of the other stabilized stage is determined by voltage eg3 which may be varied or preset by an operator at the master control station for a spread of geophones or which may be varied automatically as a function of time during the interval following a shot generally as disclosed in U. S. Letters Patent 2,408,001 to Shimek et al. With the switch 28 in the #1 position, the gains of both stages of the amplifier for geophone 22 are controlled by variation of aforesaid voltage eg3. For reasons discussed in explanation of the basic circuits, Figs. 4 6, the variations of the gain control voltages E93 and eg3 do not appear as spurious signals in the output of the geophone amplifier channel because of the stabilizing action of tubes I6 and I 6A.

1t shall be understood the invention is not limited-to the particular arrangements specilically described but that changes and modifications within the scope of the appended claim may be made.

What is claimed is:

A variable gain amplifier stage for maintaining seismic signals at desired amplitude without producing spurious signals, which comprises two pentode tubes, a source of seismic signals connected'to the control grid of a first of said tubes to vary the control grid-anode transconductance of said first tube, said rst tube having an outp ut circuit including a balanced bridge of the four-impedance type, the anode of said first tube and the screen grid of the second of said tubes being connected across a diagonal of said balanced bridge, an amplifier output circuit including a capacitor connected to the other diagonal of said balanced bridge, a source of B- supply for the anode of said first tube and for said screen grid of said second tube connected between their respective cathodes and to said last-named diagonal on the side of said bridge opposite said capacitor, a conductive connection between the screen grid of said first tube and the anode of said second tube, a conductive connection between the suppressor grids of said tubes, and means for deriving from said amplifier output circuit a gain control voltage including a gain control circuit connected to said suppressor grids to vary the suppressor gridanode transconductance of both of said pentodes, the circuit components of said bridge beductance'ofsaid fs'ttubeanduponcocurr'et V opposite' and proportionate chang iri'suppressor me of this. emi", ,4 gridscreengrid transconductan'ce of Asaid second UNITED STATESf PATENTS mbe- Y .K Number ame l, eg.

- GEORGE B LOPER- 2,360,466 xauedforat` a1. ;hfoenv 17, y1944 

